The present invention relates generally to the art of magnetic resonance. It finds particular application to magnetic resonance imaging receiver coil systems having detachable, relocatable, and/or interchangeable sections. It will be appreciated, however, that the present invention is also applicable to the examination of other portions of the human anatomy and to the imaging or spectroscopic examination of non-human subjects or other objects, materials, and so forth.
Conventionally, magnetic resonance imaging systems generate a strong, uniform, static magnetic field in a free space between poles or in a bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object to be imaged placed therein. The polarized object then possesses a macroscopic magnetic moment vector pointing in the direction of the main magnetic field. In a superconducting main annular or bore magnet assembly, the static magnetic field B0 is generated along a longitudinal or z-axis of the cylindrical bore.
To generate a magnetic resonance signal, the polarized spin system is excited by applying a magnetic resonance signal or radio frequency field B1 perpendicular to the z-axis. The frequency of the magnetic resonance signal is proportional to the gyromagnetic ratio xcex3 of the dipole(s) of interest. The radio frequency coil is commonly tuned to the magnetic resonance frequency of the selected dipole of interest, e.g., 64 MHZ for hydrogen dipoles 1H in a 1.5 Tesla magnetic field.
Typically, a radio frequency coil for generating the magnetic resonance signal is mounted inside the bore surrounding the sample or patient/subject to be imaged. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis back toward alignment, the precessing magnetic moment generates a magnetic resonance signal which is received by the radio frequency coil in a reception mode.
For imaging, a magnetic field gradient coil is pulsed for spatially encoding the magnetization of the sample. Typically, the gradient magnetic field pulses include gradient pulses pointing in the z-direction but changing in magnitude linearly in the x, y, and z-directions, generally denoted Gx, Gy, and Gz, respectively. The gradient magnetic fields are typically produced by a gradient coil which is located inside the bore of the magnet and outside of the radio frequency coil.
Conventionally, when imaging the torso, a whole body radio frequency coil is used in both transmit and receive modes. By distinction, when imaging the head, neck, shoulders, or an extremity, the whole body coil is often used in the transmission mode to generate the uniform excitation field B1 and a local coil is used in the receive mode. Placing the local coil surrounding or close to the imaged region improves the signal-to-noise ratio and the resolution. In some procedures, local coils are used for both transmission and reception. One drawback to local coils it that they tend to be relatively small and claustrophobic.
One type of local frequency coil is known as the xe2x80x9cbirdcagexe2x80x9d coil. See, for example, U.S. Pat. No. 4,692,705 to Hayes. Typically, a birdcage coil is cylindrical and comprises a pair of circular end rings which are bridged by a plurality of equi-spaced straight segments or legs. Birdcage head coils are capable of providing a high signal-to-noise ratio (SNR) and achieving readily homogeneous images. Birdcage coils are widely used for functional MRI (fMRI) and other applications.
Birdcage coils, however, are not without their disadvantages. Since, generally, the SNR and thus image quality increases with decreasing distance between the receiver coil and the volume being imaged, birdcage coils are generally designed so that they will be located very close to the subject""s head, particularly since fMRI applications require the ability to extract small signals (e.g., reported to be as low as about 2-5% at 1.5 T). As the name implies, birdcage coils are also closed or cage-like in nature and thus restrict access to the subject""s face and head. This results in a lack of space for placement of stimulation devices that would be desirable for fMRI experiments. Stimulation devices are devices constructed to stimulate a specific neural function of a subject, the response to which is sought to be observed through imaging the appropriate region of the brain. Such stimulators may emit, for example, mechanical, electrical, thermal, sound, or light signals designed to stimulate the neural activity of interest. The neural activity is induced by sensory stimuli, such as visual, auditory, or olfactory stimuli, taste, tactile discrimination, pain and temperature stimuli, proprioceptive stimuli, and so forth. Since the birdcage design is close fitting and not particularly open in nature, many such stimulation experiments must be performed in a manner that is suboptimal, if at all. For example, the use of a birdcage coil might preclude, due to space constraints, the use of an auditory stimulation device, such as a headphone set. Likewise, since bars are placed over the face, and in some instances directly over the eyes, birdcage coils are particularly disadvantageous for eye-tracking experiments or other visualization experiments.
Another problem with birdcage coils is that the design limits access to the patient, e.g., for therapeutic, physiological monitoring, and patient comfort purposes. Access may be needed, for example, to monitor physiological functions, such as oxygen levels, or to perform interventional medicine or use life-support devices, such as ventilator tubes, tracheotomy tubes, etc., while imaging a patient. Drug delivery, contrast agent delivery, and delivery of gases such as anesthetizing gases, contrast-enhancing gases, and the like, also require patient access. Also, it is also often desirable to enhance patient comfort through the use of patient comfort devices. However, the proximity of the axial segments to one another and to the head of the patient impairs such practices.
Yet another problem with birdcage head coils is their claustrophobic effect on patients. Many pediatric and adult patients already experience claustrophobic reactions when placed inside the horizontal bore of a superconducting magnet. Placement of a close-fitting head coil having anterior legs which obstruct the direct view of the patients further adds to their discomfort. Attempts to reduce the discomfort have been made, for example, through the use of illumination inside the magnet bore, air flow, and the use of reflective mirrors. Although claustrophobic reactions and discomfort are sometimes reduced somewhat by such measures, claustrophobia can still be problematic.
Birdcage coils are circularly polarized. Removing or altering the spacing of the legs adjacent the face alters the symmetry and can degrade performance.
Other types of localized coils include a phased array of smaller surface coils. In this manner, a greater SNR (that increases in proportion to the number of elements) than birdcage design can be achieved. For fMRI applications, flexible coil arrays can be wrapped around the head. However, these so-called flex-wrap designs are lacking in the spatial openness necessary for stimulation studies, interventional imaging, and the accommodation of therapy devices. Furthermore, it is difficult to achieve uniform placement of coils, both as between different subjects and for repeat studies of the same subject.
The present invention provides a new and improved localized RF coil that overcomes the above-referenced problems and others.
In one aspect, the present invention provides an radio frequency (RF) coil system for magnetic resonance imaging of one or more regions of a subject. The coil system includes a first coil section comprising one or more conductive coils in a first nonconductive housing, and a second coil section comprising one or more conductive coils in a second nonconductive housing, wherein the first and second coil sections are configured to be inherently decoupled or have minimal coupling. The coil system further comprises one or more fasteners removably and movably joining the housings of the first and second coil sections.
In a further aspect, the present invention provides a magnetic resonance imaging system comprising a main field magnet for generating a temporally constant magnetic field along a main field axis and an RF coil system. The RF coil system includes a first coil section configured for maximum or predominant field sensitivity along a first axis perpendicular to the main magnetic field axis, a second coil section configured for maximum sensitivity along a second axis perpendicular to the first and main magnetic field axes, and a fastening system for selectively fastening the first and second coil sections on opposite sides of a region of interest for quadrature reception of resonance signals emanating from the region of interest.
In yet a further aspect, the present invention provides a magnetic resonance method comprising the steps of establishing a polarizing magnetic field in a region of interest; exciting resonance of selected dipoles in the region of interest to generate magnetic resonance signals; and concurrently receiving the magnetic resonance on one side of the region of interest with a first linear coil having a maximum sensitivity along a first axis orthogonal to the polarizing magnetic field, and on an opposite side of the region of interest with a second linear coil having a maximum sensitivity along a second axis orthogonal to both the polarizing magnetic field and the first axis.
One advantage of the present invention is that it increases spatial openness around the subject.
Another advantage resides in its ability to easily select the desired coverage.
Another advantage resides in improved accommodation for stimulation devices, such as the type used for fMRI experiments, and coil placement or removal options to maximize patient comfort.
Another advantage resides in improved accommodation of patient comfort devices.
Another advantage of the present invention is that detaching coil sections still permit the remaining coil sections to be operational.
Another advantage of the present invention is that it accommodates life support or therapeutic devices such as ventilator tubing, tracheotomy tubes, immobilization collars, etc.
Another advantage of the present invention is that it provides detachable and/or relocatable coil sections matched to fMRI experiments, such as auditory or visual fMRI experiments. In addition to providing openness in the space around the subject""s head that matches the requirements of the particular fMRI procedure, data acquisition throughput is increased in that the region of interest can be tailored to the appropriate region of the brain, i.e., the region or regions containing the neural activity of interest.
Another advantage of the present invention is that aliasing can be reduced by reduction of coverage of the excitation area.
Yet another advantage of the present invention is that coil concentration can be increased for areas of interest or extended to areas not well covered by the current coil designs.
Another advantage is that switching between different fMRI experiments and/or different stimulation equipment, such as between vision and auditory experiments, can be more readily performed.
Still another advantage resides in its ability to monitor relatively small signals, such as in blood oxygen level dependent contrast (BOLD) studies.
Yet another advantage of the present invention is that it allows a technologist to readily position and lock non-imaging devices.
Still another advantage of the present invention is that it is readily adaptable to time-saving techniques where temporal resolution is desired.
Still another advantage of the present invention is its adaptability to subjects having different body shapes and sizes, including subjects for whom the conventional head coil designs might provide an ill fit.
Yet another advantage of the present invention is that it is can also be used for interventional imaging.
Another advantage is that it allows addition, removal, and exchanging of coils.
Other advantages include the improved physiological monitoring, improved drug and contrast agent delivery, and improved delivery of gases such as anesthetizing gases or contrast-enhancing, e.g., hyperpolarized, gases.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.